210 THE PRESSURE OF EADIATION AND CABNOT'S PRINCIPLE [379
If we assume from electromagnetic theory that
P = W, .................................(32)
it follows at once that
Uacfr, .................................(33)
the well-known law of Stefan.
In (31) if p be known as a function of 6, U as a function of 6 follows immediately. If, on the other hand, U be known, we have
and thenceow occupies a larger space. Similarly a rise of temperature accompanies adiabatic contraction. In the second kind of operation (2) the expansions and contractions are isothermal—that is, without change of temperature. In this case heat must pass, into the black body when the volume expands and out of it when the volume contracts, and at a given temperature the amount of heat which must pass is proportional to the change of volume.